GI Drug Could be Useful for Equine Eye Exams (AAEP 2012)

The research team believes NBB could represent an additional option for transient mydriasis--pupil dilation--in some horses.

Photo: Anne M. Eberhardt/The Horse

Occasionally veterinarians stumble across a drug side effect that's more useful than detrimental. Take, for example, the antispasmodic N-butylscopolammonium bromide (NBB), marketed in the United States as Buscopan (Boehringer Ingelheim) to treat horses with colic. As it turns out, this drug could be useful for helping veterinarians examine horses' eyes.

"One side effect of NBB is mydriasis--dilation of the pupil of the eye," explained Joanie Palmero, DVM, Dipl. ACVIM, formerly of the University of California, Davis (UC Davis), Veterinary Medical Teaching Hospital. She and UC Davis colleagues examined NBB's potential for ocular exam use, and she presented their research results at the 2012 American Association of Equine Practitioners' Convention, held Dec. 1-5 in Anaheim, Calif.

Currently, topical tropicamide remains the gold standard for routine ocular exams. But the research team believed NBB could represent an additional option for transient mydriasis in some horses.

To determine its suitability for this use, the research team administered the following four treatments (interspersed by a two-week washout period) to six healthy adult horses: topical tropicamide (positive control, as it's normally administered), topical NBB, intravenous NBB (the administration route used to treat colic), and topical or intravenous saline (negative controls).

The researchers measured pupil diameter, pupillary light reflex, and various other biologic parameters (e.g., heart and respiratory rates) from time of administration and at least 60 minutes after administration.

Palmero and her colleagues determined the following:

No changes were observed in horses treated with either topical or intravenous saline, as expected;

Topical tropicamide resulted in dilation, allowing a complete exam in all horses. Mean time to dilation was 36.7 minutes, and dilation lasted an average of 360 minutes;

Topical NBB caused the pupillary dilation of one blue iris in one horse;

Intravenous NBB allowed for dilation and complete exam in two horses (mean time to dilation was 25 minutes, and dilation lasted an average of 30 minutes);

Intravenous NBB also caused partial dilation and incomplete examinations in two horses, and caused no effect in two horses; and

A transiently increased heart rate was evident in all horses following IV NBB administration.

Palmero, who is currently practicing at Alamo Pintado Equine Medical Center, in Los Olivos, Calif., also noted that the color of a horse's eye can influence the way the eye responds to mydriatics--a phenomenon well-documented in human medicine.

"Interestingly, the mydriatic effect of NBB seemed to be more profound in the horses with blue or heterochromatic irises (exhibiting more than one color in the iris) than in horses with uniformly brown irises," Palmero noted.

She added, "These findings suggest that NBB can result in ocular dilation in some horses; however, further studies are warranted to account for breed, gender, and color differences and to determine a safe, yet reliable, NBB dose that results in dilation in a majority of horses."

In summary, while tropicamide remains the gold standard, this preliminary experiment shows interesting, potentially promising results for using NBB in ophthalmologic examinations. Additional research is needed.

About the Author

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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